In the course of an inflammatory event, triggered human neutrophils discharge and oxidatively activate two latent metalloproteinases, collagenase and gelatinase, whose ability to attack interstitial as well as basement membrane collagens arms the cells with the ability to degrade critical components of the extracellular matrix. Given the strictly defined substrate specificities of these enyzmes, the tissue-destructive potential of the neutrophil metalloproteinases is believed to be solely dependent on their collagenolytic activities. However, human neutrophils have recently been demonstrated to express a third metalloproteinase activity that could potentially exacerbate inflammatory tissue damage by inactivating alpha-1-proteinase inhibitor (alpha1PI), the primary plasma inhibitor of neutrophil elastase. At present, it is unclear whether neutrophils possess a heretofore unsuspected third metalloproteinase or whether the substrate specificity of collagenase/gelatinase unexpectedly extends to noncollagenous substrates. In either case, the fact that neutrophil collagenases and alpha1PI each belong to respective superfamilies of structurally related matrix metalloproteinases and serine proteinase inhibitors (i.e., Serpins), raises the possibility that metalloproteinases play a more general role in inflammatory events than previously appreciated by mediating the hydrolytic inactivation of those antiproteinases responsible for regulating complement and contact system activation, fibrinolysis and connective tissue turnover. In order to clearly define both the pro-inflammatory potential neutrophil metalloproteinases as well as that of the distinct metalloproteinases synthesized by other cell types, the following five aims will be addressed. First, to identify the neutrophil metalloproteinase(s) that mediate(s) alpha1PI hydrolysis; second, to determine the range of Serpins susceptible to metalloproteinase-dependent proteolysis; third, to identify metalloproteinase inhibitors capable of regulating the activity of the Serpin-cleaving enzyme(s) released from neutrophils; fourth, to determine the ability of structurally-distinct metalloproteinases isolated from connective tissues to exert anti-Serpin activities; and fifth, to describe the biological consequences of Serpin inactivation. The characterization of Serpin-cleaving metalloproteinases, their substrate specificity, regulation and tissue distribution should not only provide new insights into the pathophysiology of the inflammatory process, but also identify new targets for pharmacologic intervention in a diverse array of disease states.